132 DR. THOMAS STERRY HUNT ON THE 



York system. Tiie subsequent studies of Logan in western Vermont, as given by him in 

 1863, showed that these ancient rocks are brought up by a north and south dislocation, 

 with upthrow on the east, from beneath rocks of Trenton, of Chazy, or of Levis age, which 

 latter here occupy their natural position at the summit of the Upper Taconic or First Gray- 

 wacke group." Billings, also in 1868, as already pointed out, had shown that farther 

 southward in Vermont the Red Sand-rock, or Lower Potsdam, is in like manner brought 

 up by a dislocation, so as to overlie on the east the Loraine shales. 



§ 154. It now became clear that much of what had been called Hudson River group, 

 to the east of the Hudson Valley, and of Lake Champlain, consisted, not as taught by 

 Mather and his foUovv^ers, of disturbed and altered strata newer than the Trenton lime- 

 stone, and of the age of the Loraine shales, but of older rocks, carrying in part, at least, the 

 forms of the first fauna. We have already seen (§ 112) how, in view of these facts, Hall 

 expressed his opinion in 1802, as to the relations of these newer strata to the older ones. In 

 18Ï7, he returned to the subject and, after retracing the history of investigation, concluded 

 that " we now know approximately the limits between the newer and the older forma- 

 tions, and there is now no longer any question that the newer series, or the rocks above 

 the Trenton limestone, do occupy both sides of the Hudson River for nearly one hundred 

 miles, and continue along the valley for many miles farther towards Lake Champlain. The 

 term, Hudson River group, has, therefore, a definite signification, from absolute knowledge 

 of superposition and fossil remains. The error lay in extending the term to rocks on the 

 eastward, at a time when their fossil contents had not been studied, and were, in fact, un- 

 known, and their geological position had not been determined by critical examination." '" 

 We have already shown, in §§ 13-14, how Vanuxem had devised this term to include, 

 besides the true Loraine shales, other disturbed and apparently non-fossiliferous rocks of 

 controverted age, which he supposed might be included with the former, and thiTs intro- 

 duced much of that confusion which has iirevailed in the use of the name of Hudson River 

 group as the equivalent to that of Loraine shales. 



^ 155. The eastern limit of the rocks of the second fauna, along the Hudson valley, being 

 defined, as stated by Hall, and as already shown by him for that region on Logan's geolo- 

 gical map previously published, it was important to determine the age of the iincrj^stal- 

 line rocks along their eastern border, and to decide whether these were, (as mapped by 

 Logan), portions of the so-called Quebec group, or of the still older Potsdam, which had been 

 foiTud in this position at several points in Vermont. Nothing has contributed more to the 

 solution of this problem than the careful studies of Mr. S. W. Ford, who, in 18Y1, discovered 

 the existence of fossiliferous rocks of this lower horizon at Troy, New York, and, following 

 up his investigations, showed that these strata, containing an abundant fauna of Lower 

 Potsdam age, (corresponding to the Olenellus slates of Georgia, Vermont, and to the beds 

 at Bic, Quebec, and at the Strait of Belleisle, in Labrador,) are at Troy brought up on the 

 eastern side of a fault, against the Loraine shales." Continuing his studies. Ford has re- 

 cently traced these Lower Potsdam rocks, under similar conditions, through various parts 

 of Columbia and Duchess Counties, the stratigraphical break and the upthrow of the 

 Cambrian strata on its eastern side being well defined. He does not attempt to estimate 



' Geology of Canada, chap, xxii, pp. 844-860. 



" Hall, Proc. Amer. Assoc. Adv. Science, 1877, p. 263. 



" Amer. Jour. Science, 1873, vi, p. 135. 



